Internal combustion engines include engine control systems that use information collected from an engine's exhaust to monitor and control the operation of the engine. For example, oxygen sensors are included in the exhaust systems of internal combustion engines and provide information regarding the ratio of air to fuel being supplied to the engine. In general, internal combustion engines need a specific air-to-fuel ratio (or ratio range) to operate correctly. When the ratio is less than desired, not all fuel in the air-fuel mixture is burned or combusted. This situation is referred to as a rich mixture or rich condition and has a negative impact on exhaust emissions. When the air-fuel ratio is higher than desired, excess oxygen is present in the air-fuel mixture. This situation is referred to as a lean mixture or lean condition. When an engine burns lean, engine performance may decrease and, in some cases, may cause engine damage and have a negative impact on exhaust emissions.
Internal combustion engines may also produce particulate emissions. For example, the exhaust produced by a diesel engine may include a significant amount of carbon particulate (soot). The exhaust system may include soot sensors that measure the amount of soot being output, so that the engine control can reduce the emission values, or apply maintenance operations to devices that remove the soot from the exhaust.